This invention relates to gas tube lasers and, in particular, to CO2 lasers utilizing a combined discharge in static and alternating electric fields.
Gaseous lasers have found extensive applications in the laser processing industry, including laser cutting, welding of materials, laser hardening through phase transformation, and in medical applications. In particular, in recent years, there has been considerable investigation into various forms of carbon dioxide gas (CO2) lasers, which radiate at wavelengths between 9 and 11 xcexcm, and may be operated in CW (Continuous Wave) or pulsed regimes. While other gas lasers have efficiency of 0.1% or less, the CO2 laser may have an efficiency up to about 30%.
For excitation of the CO2 lasers (both CW and pulsed), it is known to utilize DC (direct current) continuous electric discharge and/or RF (radio frequency) alternating electric discharge. In conventional cylindrical excited lasers, electric discharge is usually applied longitudinally between DC electrodes disposed at opposite ends of a laser tube, whilst radio frequency (RF) discharge is normally applied across the transverse dimension of the laser tube. On the other hand, in conventional slab lasers only RF discharge is used that is normally applied between the slab electrodes.
The possibility to increase the CW output power of a cylindrical DC and/or RF excited CO2 laser by the design of its geometry is essentially limited, inter alia, due to its output power scaling. In particular, the output power of such a laser may be increased only by increasing the length of the laser cavity and it cannot be increased by increasing the inner diameter thereof due to temperature limitations.
As far as pulsed DC excited cylindrical lasers are concerned, the output power of such lasers cannot be increased by increase of either the laser length or the laser diameter. This is mostly due to their thermal instability associated with the localization of the plasma discharge to a small portion of the inner cavity volume, exciting acoustic oscillations, producing high temperature electrons which decompose the CO2 molecules, whereby the laser beam quality and its pointing stability are deteriorated. These phenomena essentially limit manufacturing of powerful DC excited lasers having compact sizes.
On the other hand, the output power of an either CW and pulsed slab RF excited CO2 laser may be increased, inter alia, by the increase of the inner area of the laser slabs. This feature is essential for fabrication of compact powerful lasers. In addition, by virtue of the use of transverse RF discharge for CO2 laser excitation quite uniform and stable discharge may be produced. However, high power RF operation requires more expensive components, which increase the cost of the transverse RF discharge excitation if high peak powers are required. An utilization of high power RF operation is also limited by a phenomenon that is known as transition of the plasma to xcex3-discharge.
It has been suggested in the prior art to utilize combined DC and RF discharges for the excitation of CO2 cylindrical lasers. For example, U.S. Pat. No. 5,097,472 describes a laser in which the same electrodes are used in-turn for the application of the RF and DC discharges. Further, U.S. Pat. No. 5,596,593 discloses a CO2 laser in which different DC and RF electrodes are used, and the RF discharge is applied orthogonally to the DC discharge, both discharges being directed transversely to the longitudinal axis of the laser. Another yet laser apparatus also utilizing the combination of DC and RF discharges is described by Yatsenko N. A. xe2x80x9cGas Discharge Lasers with Combined Pumping,xe2x80x9d Gas Laser-Recent Development and Future Prospectsxe2x80x9d, 1996, pp. 135-154. In the latter laser, a primary DC electric discharge is spread over the laser tube longitudinally between a pair of electrodes disposed adjacent the opposite ends thereof. The RF discharge is applied transversely along the diametric dimension of the laser tube. Such configuration of the laser results, inter alia, in the reduction of consumed RF power at the expense of the increased consumption of cheaper DC power, and also in the increase of laser efficiency, and in the improved uniformity of the excitation of the gas medium.
The prior art cylindrical lasers utilizing a combined DC and RF discharge operate with optically stable resonators that normally comprise, disposed at opposite ends of a laser cavity, a highly reflective output mirror which functions both to reflect internal radiation beams into the laser cavity and to transfer an output radiation beam exiting out of the laser cavity, and a feedback mirror. The two mirrors allow the internal beams to numerously oscillate inside the laser cavity in order to get high gain and improved directionality of the output beam. However, if the output beam is too intense (laser power larger than 2 kW), the output mirror may suffer breakage that may cause shut down of the beam production.
In addition, a problem exists with the use of optically stable resonators in lasers where high output power is achieved by the increase of the laser tube inner diameter. Namely, it is known that an optically stable resonator operates in a multi-mode regime and produces a low quality laser beam, when its Fresnel number NF=xcex12/(xcexL) exceeds the value of 3, where xcex1 is a radial dimension of an exposed output mirror surface, xcex is the wavelength of radiation inside the resonator, and L is the resonator length.
It has been known in the prior art to provide a powerful laser equipped with an unstable optical resonator with a relatively large Fresnel number (NF greater than 3). The unstable resonator has primary and feedback mirrors, wherein the primary mirror is of a larger diameter than the feedback mirror so that the output radiation reflected from the periphery of the primary mirror is directed out of the laser cavity in a ring shaped beam surrounding the feedback mirror. The unstable resonator produces high optical quality beam, which may extract energy out of the entire gain volume. Furthermore, in view of the fact that the number of times the laser beam passes the laser cavity is small, the use of optically unstable resonators requires a specific care to be taken of the gain in the laser medium.
It is generally known that, in a gas laser having an unstable resonator, the gain may be enhanced by the increase of the gas pressure. However, usage of high pressure in pulsed lasers normally decreases the pulse repetition frequency since the gas needs a relatively long time in order to recover. Hence, conventional pulsed lasers having an unstable resonator cannot operate with high pressure and, therefore, pulses provided thereby are normally of relatively small averaged power, that inevitably limits their applications.
It is the object of the present invention to provide a new gas laser.
In accordance with the present invention, there is provided a cylindrical laser apparatus comprising:
(a) a laser chamber including an elongated discharge region extended along a longitudinal axis of the laser apparatus and containing an active medium to be excited in the discharge region so as to emit photons of induced radiation;
(b) a pair of DC electrodes facing toward the discharge region and arranged at opposite ends thereof, for the provision of a longitudinal DC discharge in the discharge region;
(c) a pair of RF electrodes facing toward the discharge region and arranged alongside the discharge region, for the provision of a transverse RF discharge therein;
(d) an unstable resonator including a primary mirror and a feedback mirror, the mirrors being disposed at said ends of the discharge region along its axis for forming an outlet beam of the radiation induced by the DC and RF discharges.
The laser apparatus in accordance with the present invention is preferably a CO2 laser.
Utilizing the combination of DC and RF discharges in the laser apparatus of the present invention, high power output radiation in general and, particularly, high power pulsed radiation may be provided with a relatively compact design of the apparatus. Consequently, the laser operating efficiency is increased, with a significant lowering of required excitation voltages, and a substantial reduction in excitation hardware. Moreover, in addition to these advantages, as well as other advantages of the DC and RF discharge lasers mentioned heretofore, the combined use of the DC and RF discharges is capable of creating a discharge having an improved stability and uniformity and high density of the electric energy. In particular, the density of the electric field that may be obtained by the combined use of the DC and RF discharges may reach 200 W/cm3. Such a high density of the electric energy provides for an increased gain in the discharge region at medium pressures of the laser gas, enabling thereby the use of the optically unstable resonator with pulses in a rather broad region of duration of 0.05-1 msec.
The laser apparatus of the present invention may have durable and reliable construction that is relatively easy and cheap in manufacturing.